In an attempt to improve medical care and cost effectiveness in the follow-up evaluation of treated well differentiated thyroid carcinoma, we have used the radioimmunoassay of thyroglobulin. We've shown in 50 patients studied cross-sectionally that this simple blood test, in the absence of hypothyroidism, is as accurate or more accurate than total body iodine scans. In an effort to show that this blood test will be just as useful in longitudinal studies, we are in the process of comparing serum thyroglobulin levels measured every 6 months with total body iodine scans performed every year in a large group of patients treated for thyroid carcinoma.